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Equivalent Square
Thank you for your contribution to the Radiation Oncology wikibook. I noticed an error in one of your edits on the Radiation Oncology/Physics/Equations page, concerning the computation of the equivalant square for a rectangular photon field.

The correct formula for the equivalent square is $$S=\frac{2ab}{a+b}$$.

You may have been confused by the formula for area/perimeter which is indeed $$A/P = \frac{ab}{2(a+b)}$$. The equivalent square is stated as the square which has the same area/perimeter as the rectangular field.

A square has an area/perimeter of a/4 (from a^2/4a). Thus, solving for equivalent square (S), setting the A/P for the equivalent square equal to the A/P for the rectangle, you get: $$\begin{align}&\frac{S}{4} = \frac{ab}{2(a+b)} \\ &S = \frac{2ab}{a+b}\end{align}$$. This formula can also be expressed as S = 4*A/P.

Hope this helps! We encourage you to continue editing the Radiation Oncology wikibook! &mdash; Thanks, Brim (talk) 16:48, 5 October 2009 (UTC)